1. Field of the Invention
The present invention relates generally to analog and digital communication systems. More specifically, the invention relates to a system and method for emulating signal impairments which manifest themselves during signal conversion and transmission.
2. Description of the Related Art
In simple terms, a communication system transmits information from a source to a destination. The information signal is transmitted from the source to the destination over an appropriate medium which may be guided or unguided, comprising copper, optical fiber or air and is commonly referred to as the communication channel. To use the channel for transportation, the information signal must be altered to match the characteristics of the channel which is referred to as modulation. The recovery of the information-bearing signal is called demodulation. The demodulation step converts the transported signal using the logical inverse of the modulation process.
A prior art communication system is shown in FIG. 1. The communication system in FIG. 1 shows a single direction communication link for a data signal from one location (user 1) to another (user 2). The prior art system comprises a transmit modem, an up-converter or transmitter, a communication medium, a down-converter or receiver and a receive modem. The transmit modem accepts a data input and produces a modulated digital or analog baseband output according to the chosen modulation scheme. The modulated data is input to the transmitter where it is upconverted onto a predefined carrier frequency and output to the communication medium. The receiver and receive modem perform a similar inverse operation.
Deployed communication systems rarely are single access. Protocols such as time division multiple access (TDMA), carrier sense multiple access (CSMA), code division multiple access (CDMA) and frequency based protocols such as frequency division multiple access (FDMA) and orthogonal frequency division multiplexing (OFDM) allow access to the same communication channel for more than one user. These techniques can be mixed together creating hybrid varieties of multiple access schemes such as time division duplex (TDD). The access protocol specified by a communication system is typically executed after the data undergoes modulation.
Prior art modulation techniques that are in use are frequency modulation (FM), frequency shift keying (FSK), phase shift keying (PSK), binary phase shift keying (BPSK) and differential phase shift keying (DPSK). The most commonly used high-speed methods for data modulation are quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK). These techniques vary a predefined carrier frequency amplitude and phase according to an input signal to transmit multiple bits per baud thereby using available bandwidth more efficiently.
To extend the possible range of data signal values, quadrature modulation assigns a symbol to represent more than two binary values. The concept of a symbol allows for a greater degree of transmitted information since the bit content of each symbol dictates a unique pulse shape. Symbols, which encode x bits per symbol may represent a quantized version of an analog sample or digital data. Depending upon the number of symbols used, an equal number of unique pulse or wave shapes exist. The number of data bits determine the combinations of amplitude and phase that define a constellation pattern.
Quadrature modulation is based on two distinct carrier waveforms that are orthogonal to each other. If two waveforms are transmitted simultaneously and do not interfere with each other, they are orthogonal. Quadrature modulation modulates two different signals into the same bandwidth creating a two-dimensional signal space as shown in FIG. 2a. Two waveforms generally used for quadrature modulation are sine and cosine waveforms at the same frequency. The waveforms are defined ass1(t)=a1cos(2πfct)  (1)s2(t)=a2sin(2πfct)  (2)where fc is the carrier frequency of the modulated signal and a, and a2 are the amplitudes applied to the cosine and sine signals respectively. By convention, the cosine carrier is called the in-phase, real component I of the signal and the sine carrier is the quadrature, imaginary component Q of the signal. Linear combinations of the form a1 cos(2πfct)+a2 sin(2πfct) (where a1 and a2 are real numbers) generated from the two basic waveforms define symbols in the modulation alphabet. The symbols can be represented as complex numbers, a1+ja2, where j is defined as j=√−1.
A QAM symbol consists of at least one sample from both the in-phase I and quadrature Q signals. Signal amplitude is indicated by the distance from the origin; phase by the angular distance around the unit circle. After the data is assembled as symbols, the symbols are further processed in accordance with the access protocol chosen for the communication system.
The above processing is typically performed in a modem. Afterwards, a predefined carrier frequency is modulated with the baseband output from the modem, amplified and transmitted in the communication medium. Upconversion is required when the channel frequencies are above baseband frequencies. Transmission through a medium is accomplished by converting the modem output signal amplitude, frequency or phase to an operating region between 104 to 108 Hz using radio frequency amplifiers, 108 to 1011 Hz using microwave frequency amplification and 1011 to 1016 Hz using optical frequency amplification. Reception of the communication transmission is by downconversion.
Modulation schemes that rely upon phase must overcome the inevitable problem of phase synchronization. For proper signaling, the I and Q channels should have the same gain when processing both received channels, keeping the I and Q channels uncorrelated. Mismatched signal gains or magnitudes between the uncorrelated I and Q channels create errors when processing. Phase differences other than 90 degrees between the carrier waveform signals cause spillover between individual channels and similarly result in degraded performance. However, during carrier modulation (upconversion), transmission through the communication channel and carrier demodulation (downconversion), signal impairments occur.
Signal impairments which manifest themselves during the conversion processes are gain and phase variations in the separate I and Q channels. This is due in part to the plurality of electronic mixers, filters, A/D converters, etc., employed in the design of up and downconverters. Each component contributes its own variation in specified value due to temperature, manufacturing tolerances and other factors until the variations taken as a whole significantly affect signal integrity. Amplitude and phase imbalance between the I and Q channels result in the constellation distortions shown in FIGS. 2b and 2c, decreasing overall signal-to-noise ratio (SNR).
Of these impairments, amplitude and phase impairments are linear distortions. Other significant linear impairments which manifest themselves in a data signal during carrier frequency modulation and demodulation comprise: carrier frequency offset, caused by local (receiver) oscillator drift; carrier phase noise, impressed on the data signal by active devices in the signal path; communication channel bandwidth aberrations, caused by unintentional filtering; group delay variation and carrier amplitude imbalance.
Non-linear impairments are another adverse byproduct. Non-linear distortions are characterized by changes in output gain or phase which vary in dependence upon the input signal magnitude. The two major signal impairments include: amplitude-to-amplitude (AM—AM) distortion caused by non-linearities in the overall amplifier gain transfer function and amplitude-to-phase distortion (AM-PM conversion) distortion caused by amplitude dependent phase shifts.
In addition to the impairments brought about by up and downconversion, the communication media, whether guided or unguided, is under the influence of obstacles, attenuation and wave reflections. These perturbations affect signal level by many dB and continually change in a mobile communication operating environment. The propagation characteristics vary widely depending upon whether a communication link is fixed or mobile, the condition of the propagation path and the composition of the medium itself.
While designing and prototyping new communication systems, manufacturers routinely and thoroughly test the baseband modulation/demodulation components and the up/downconversions to and from the transmission channel operating frequencies. To validate a modem hardware design, prior art test techniques comprise signal generators, Eb/No (ratio of carrier or bit energy to noise energy) generators and meters, channel emulators, etc. However, this method does not include the conversion components.
The prior art testing method suffers from two fundamental disadvantages. First, the method is not capable of evaluating a design at the baseband signaling frequencies since up/downconversion and transmission channel impairments are difficult to separate from algorithmic or other systemic deficiencies. Second, the prior art does not provide a modem-to-conversion and transmission medium evaluation interface prior to integration with actual hardware.
Accordingly, there exists a need for a system and method that allows for the evaluation of a complete transmit modem-to-receive modem system by simulating in the baseband impairments in signal quality manifested during signal conversion and within the transmission medium.